High pressure metal vapor discharge lamps such as high pressure mercury vapor discharge lamp have been used for various usages for interior and exterior lighting. However, from the viewpoints of necessity of saving of energy, the power savings have been considered to turn off certain percentages of lamps in comparison with the lighting lamps in past. Such thinned-out lighting causes an irregular intensity of illumination whereby a trouble may be disadvantageously caused in workings and it is not preferable in practice.
In order to save power for lighting, the uses of lamps having higher efficiency can be considered. However, in such case, a construction for exchanging the lighting devices required is costly. Even though construction for the new lighting equipment is not required and only lamps are exchanged, needless intensity of illumination may be given or coloring of the new lamps may be remarkably different whereby a trouble may be caused in workings.